Conventionally, the fabrication of tooling die inserts requires highly skilled tool and mold makers using computer numerical control (CNC) machining and electrical discharge machining. Hence, a tooling die insert for a complex plastic part such as a radio housing can typically take several months and over one-hundred thousand dollars to fabricate using the conventional methods. In addition, tooling die insert detail is limited by machining constraints, therefore if intricate surface textures are desired in the end-molded product, they are added to the tooling die insert using time-consuming and labor intensive masking and etching processes. Finally, machined tooling dies are generally composed of one material having homogenous material properties throughout, and therefore the material is usually chosen as a trade-off of optimal mechanical strength, machinability, wear resistance and heat transfer capability.
Due to the market demand for reduced cycle time, several processes have been developed that fall under the guise of rapid tooling. The unifying factor of each of the various rapid tooling techniques is that they all utilize a rapid prototype (RP) pattern of some sort with a metal fabrication process to create a tooling die insert. Some rapid tooling methods have been developed based on the metal fabrication process of investment casting. Using an expendable pattern as a guide, investment casting creates a ceramic shell into which molten metal is poured to form the final part. 3D Systems, Inc. commercialized a technique known as QuickCast which uses a stereolithography master as the expendable RP pattern. Soligen Inc. uses an automated droplet deposition method to build the ceramic shell directly by selectively applying an adhesive to a powdered bed that forms each cross section into a composite-like material. Wax or plastic patterns produced by other RP systems, such as DTM Corp.'s Selective Laser Sintering, Stratasys's Fused Deposition Modeling and Sanders Prototype's ink-jet printing can also be used with the investment casting process for creating the ceramic shell. Each of these investment casting based techniques are an improvement over the conventional machined approach in regards to cycle time and cost. In addition, using the investment casting technique it is possible to incorporate conformal cooling channels directly into the tooling die insert as opposed to drilling the cooling channels. Drawbacks to the investment casting techniques include poor surface finish, geometry limitations of the ceramic shell and investment casting process, and extreme hardness of the cast tooling die inserts.
Other prior art rapid tooling methods are based on a `metal shell` approach, in which a metal shell is first formed onto the rapid prototype pattern, removed, and reinforced to form a tooling die insert. U.S. Pat. Nos. 4,726,412 and 5,189,781 describe techniques for rapidly creating tooling die inserts by spraying metal onto a pattern. Cemcom Corp. is commercializing a process in which nickel is rapidly electroformed over an RP pattern. High strength ceramic is then used to bond the metal shell to a standardized pocketed mold base. The Marshall Space Flight Center has demonstrated the use of vacuum arc vapor deposition to apply the metal shell to the RP part. Each of these techniques allow for the production of a tooling die insert very rapidly and at low cost. However, the tooling die inserts produced by these methods generally have very poor thermal conductivity, are susceptible to rapid wear and hence short useful life, and have process related geometry limitations.
Powder metal based rapid tooling methods use varying techniques to form powdered metals and a binder into a tooling die insert `green part`, which is then placed in a furnace that removes the binder and sinters the powders together. Finally, the fused part is densified via infiltration with a low melting point metal such as copper. Keltool is a process developed by 3M Corp. which uses an RP pattern to make a silicone mold which is filled with finely powdered A6 tool steel, tungsten carbide and an epoxy binder which, after curing, forms the green part. DTM Corp. developed the RapidTool process which uses selective laser sintering to form the green part from low-carbon steel powder coated with proprietary additives and carried in a proprietary binder system. Three-dimensional printing, an RP technology invented by Emanual Sachs and Michael Cima of the Massachusetts Institute of Technology, uses electrostatic ink jets to selectively spray a colloidal acrylic binder onto stainless-steel powder to create the green part. Advantages of the powder metal based rapid tooling methods are speed, low cost, and the ability to create complex geometry and incorporate conformal cooling channels into the tooling die inserts. Disadvantages include poor surface finish and warpage due to sintering and infiltration.
Although the rapid tooling technologies have reduced the cycle time from several months to a few weeks, a need still exists for an even faster process for creating tooling die inserts that reduces the cycle time down to one or two days. The new process should be able to create tooling die inserts having complex geometry with mechanical properties able to withstand the pressures and temperatures of the injection molding process. The new process should also be able to incorporate conformal cooling channels and integral surface texture. Further, the new process should allow for multiple materials to be utilized to optimize various portions of the tooling die insert; i.e., high wear material at the cavity surfaces and high thermally conductive material about the cooling channels.